72
HIGH SPEED SPINNING OF VISCOSE FILAMENT YARNS
Martin Nywlt
ENKA GmbH & Co. KG
e-mail: Martin.Nywlt@enka.de
ENKA invested in the technology of high
and shrinkage, are excellent. Also at
speed spinning to be prepared for the various
manifolded spinning speed, the special ENKA
developments in the textile market.
continuous spinning process shows the
The continuous spinning process according
advantages in product properties in
the ENKA CHEV-technology can now be
comparison to the Nelson-type technologies.
operated at 500 m/min spinning speed. At this
Further increase in spinning speed is limited
speed, the production stability and the
by mass-transfer in the yarn forming step and
product properties are at an optimal level.
the washing units. Technically, a spinning
Especially the level and the constancy of all
above 1000 m/min is possible, but ENKA sees
important yarn properties, such as dye uptake
no economical benefit at this high speed.
__________________________________________________
Introduction
The viscose technology is the origin of the
man-made fiber industry. The current basic
technologies were developed in the years 1950
to 1960. These technologies are known as
• Spool process
• Pot – or centrifuge process
• Continuous process.
For ENKA, it was the target to clarify the
potential of all of these technologies. The
discontinuous processes are highly optimized.
At our plant Elsterberg, the most modern spool
process was built in 1991 to 1993, by utilization
of all optimization potential of the individual
process stages.
This paper will show the options and
possibilities of the continuous technology.
Target of the 5-year development was to
improve the productivity by manifolding the
process speed.
Current situation : viscose filament
The development of the high-speed spinning
process started at ENKA in the early 90s, a time
of high demand for viscose filament yarns. A
first phase of restructuring was finished,
materials with natural origin were en vogue
especially in Italy.
In the years 1998 to 2000, we faced in Europe a
trend towards “TECHNO” materials, based on
polyamide and polyester. This trend leads to a
further decrease of the viscose filament capacities
worldwide. In 2002, some recession and a weak
demand in the US hit the total textile industry.
This situation already influenced the top textile
industry and will surely have impact on the yarn
industry.
As a consequence, in the past 2 years, the viscose
filament capacity worldwide decreased by more
than 30%. If the temporary weakness will last
longer, a further capacity reduction will take
place.
Quality requirements for viscose filament yarns
The textile applications for viscose filament yarns
have remained basically unchanged over the past
decades. In the apparel segments, viscose filament
yarns are used for crepe fabrics, knitwear,
embroideries and yarn blends together with wool
or synthetics.
For garment lining fabrics, viscose filament yarns
are still the most favorable materials to get the
right fabric properties.
73
Dependent on the fashion cycles, these various
segments are characterized by strong
fluctuations in demand.
A successful yarn process for these markets has
to be designed for high flexibility to cover all
these applications. The titer range has to cover
all segments; the technical performance of the
yarns has to be suitable for these various
textile-processing steps. For ENKA, which
serves all these textile segments, the basic yarn
properties
• Constant dye uptake,
• Consistency in all physical properties,
• Excellent mechanical processing properties
have to be the premises for the high-speed
spinning process. In the current textile industry,
where time to market is the ultimate
requirement, product deviations influencing the
processability or fabric characteristics are not
tolerated.
is more restricted to the heavy deniers. ENKA
therefore divested the production plants based on
pot spinning and invested in spool technology in
Elsterberg.
water
dryer
yarn
winder
yarn
Technical requirements for the high-speed
spinning process
Based on the market demands, the technical
requirements can be defined.
Constant dyestuff uptake is achieved by
constant skin-core relation and porosity. This
means constant process parameters, such as
temperatures, concentrations, and residence
time throughout all process steps.
Excellent mechanical processability requires a
low number of contacts of yarn and machine
parts, high quality of the surface of yarn guides,
godets with a controlled drive and a
safeguarded protection of the individual
filaments in all process steps.
Basic tenacity and elongation is tuned by the
spin bath and viscose solution parameters.
Comparison of existing technologies
The known processes can be divided into four
basic technologies. Spool and pot-processes are
both discontinuous types. For all the various
steps (spinning, washing, drying and winding)
the optimal processing speed and parameters
can be chosen. The disadvantage is the high
labor intensity and high processing times.
Within these two discontinuous technologies,
the spool process covers a titer range from 67
dtex to 660 dtex at top quality; the pot process
washing field
Figure 1. Nelson and ENKA CHEV principle.
The Nelson and the ENKA CHEV technology are
both continuous processes with short production
time (Figure 1). The compact Nelson process
realizes the post-coagulation step and the yarn
washing with low space requirements. This results
in comparatively low investment costs. This
benefit in investment leads to a weakness in
parameter constancy during these process steps.
Result is the broad variance in physical yarn
properties and dyeing characteristics.
The technical more sophisticated ENKA CHEV
process has a parallel yarn treatment. The different
washing steps are fully separated and can be
operated at optimal conditions. As a result, the
quality of the CHEV yarns is fully comparable to
the spool spun yarns.
74
Development of high-speed spinning viscose
filament
ENKA started the development of high speed
spinning to reduce the specific investment cost
of the CHEV process, which were clear higher
than for a Nelson process. The product portfolio
had to cover the titer range from 40 dtex to 167
dtex at the established ENKA quality level.
Especially the yarn shrinkage of max. 5 % was
to be kept beyond this limit. The target spinning
speed was defined as 500 m/min, which meant
a factor of 3 to 5 in comparison to the
established speed.
Process principle for high-speed spinning
In a continuous spinning process, the yarn
speed in every step has to be identical. The
dimensions of each unit are determined by the
necessary residence time. For a minimized
investment and therefore the optimal
economics, the target is to reduce these
residence times as much as possible. An
alternative to operate the process steps of a
continuous spinning at different speeds was
developed by Asahi. The high-speed yarn
formation was followed by a low speed
washing, where the yarn was folded on a sieve
belt. ENKA did not follow this process strategy
due to our demands in quality and process
stability.
The mass transfer mainly determines the
residence times of the processing steps. The
time limiting mass transfer is of course different
during coagulation, degradation of the xanthate,
washing and drying.
The first step, the chemical reaction, is a fast
neutralization of sulphuric acid and caustic. The
reaction rate depends only on the acid diffusion
to the reaction zone. The washing process is
only controlled by diffusion. The limiting mass
transfer coefficients of e.g. sodium sulphate in
cellulose and in the laminar film are constant.
Drying is also a diffusion process where heat
transfer and the diffusion coefficient of water in
cellulose are the determining items. Common
for these types of diffusion processes are the
limited possibilities for acceleration of mass
transfer.
Yarn Formation
The principle of yarn formation had to be
developed according a totally new approach. The
residence time for fiber formation is constant due
to diffusion of acid into the filaments. A stagnant
spin bath similar to spool spinning allows only a
spinning speed of 150-200 m/min. At higher
speed, the friction of bath to fiber is too high, so
that the yarn will break. With a spinning tube
according the CHEV technology, an increase of
speed can be achieved. Here the limits are the risk
of turbulent flow of spin bath in the tube resulting
in a kind of intermingling of the filaments. The
only way, in our experience, to spin at 500 m/min
or faster is the use of a free falling spin bath
(Figures 2 and 3).
Washing Process
The washing section gives the opportunity to
increase the mass transfer to a high extent. The
diffusion barriers of the total filament bundle can
be increased by opening this bundle. The target in
the washing section is to limit the film diffusion at
the filament.
Drying Process
Contact dryers can reach high heat transfer rates.
Other drying technologies, such as convection or
microwave, do not have the efficiency. Here,
component suppliers can offer an optimal
technology.
Pilot plant
A pilot plant with a 100 t/y capacity, engineered
by ENKA, was operated for 2 years. In this pilot
phase, all important components were tested,
various yarn types were produced and processed
according all typical viscose filament applications.
The yarn titers 40 dtex up to 167 dtex could be
spun at 500 m/min. The higher deniers required a
reduction in spinning speed due to the mass
transfer limitations. For these yarn types, the
typical ENKA product properties were reached.
The process stability at high speed is much more
demanding. Yarn tension control throughout the
total process steps is a must, requiring a
sophisticated and also robust control system of the
godet drives.
75
viscose
spinbath
yarn
spinbath
yarn
spinbath
viscose
spinbath
FALLING FILM SPINNING
TUBE ( CHEV ) SPINNING
Figure 2. Comparison of spinning.
Spinning
section
falling film
Washing section according CHEV
principle
First hot air
dryer
Second
dryer
contact
Winding
section
Figure 3. Process scheme.
Limits of High Speed Spinning
The major technical limit is mass transfer
within the bulk phase of the individual
filaments and the mass transfer restrictions of
film diffusion. An increase of spinning speed
can be achieved by a proportional increase of
• Falling film length
• Number of washing elements
• Number of drying rolls
with increasing speed, the process stability will
become the determining limit. Yarn breaks are
more difficult to control, the probability to
restart the total machine after minor problems
increases. According ENKA´s experience, the
achieved 500 m/min spinning speed is the
current optimum between efficiency and
process stability.
The increase in spinning speed by factor 4 shows
the potential of the viscose technology, which has
already a history of about 100 years. By this
development, the specific investment costs for a
continuous filament production based on ENKA´s
CHEV technology is favorable compared to the
Nelson technology. By implementing the
advanced process, the investment costs would be
economically attractive. ENKA is therefore
prepared for all possible market situations.